Extremism is the holding of extreme political or religious views, often advocating illegal, violent, or other forms of extreme behavior. Extremism is considered by many to be a necessary condition of terrorism. In many cases, terrorist groups require their adherents to adopt extremist and intolerant worldviews in order to dehumanize, persecute, kill, or oppress dissidents and opponents. Often, the victims of extremism are political, religious, ethnic, and sexual minorities.

Extremism is often divided into two categories: violent extremism and non-violent extremism. Violent extremism is used to describe movements that endorse certain forms of violence (including terrorist violence) to pursue an extremist ideological agenda. So-called non-violent extremism is used to describe movements that pursue similarly extremist agendas through non-violent means, including through a country’s preexisting democratic or political process, or through non-violent advocacy, political organizing, and protests. 

Both forms of extremism—violent and non-violent extremism—work to pursue end goals that are intolerant of dissidents and potentially violent. There are many types of extremism, including Islamic extremism, far-left extremism, far-right extremism, and forms of special-interest extremism that focus on topics such as animal rights and environmentalism. These movements—including those that profess non-violence—may turn violent in the short-term, e.g. through terrorist activity, or in the long-term, e.g. through state-sanctioned violence and human rights abuses.